When preparing a car for racing, it is often not desirable to have the front wheels exactly parallel to each other. The wheels often must be aligned by adjusting the angles of the wheels to achieve optimal handling. Toe is a measurement that determines how much the front and/or rear wheels are turned in or out from a straight-ahead position. Toe-in is a condition that occurs when a wheel is aligned such its leading edge is angled toward the vehicle's chassis centerline. Toe-out occurs when the leading edge of the wheel angles away from the chassis centerline. The amount of toe is typically given as the difference between a measurement of the distance from a tire to a reference marker at the leading edge of the tire and a measurement of the distance from the tire to a reference marker at the trailing edge of the tire, although it can also be given as the angle in which a wheel is out of parallel. Toe settings are important because they affect performance through tire wear, straight-line stability, and corner handling characteristics. Vehicles may have different toe settings depending on the impact the driver wants on directional stability. Toe-in enhances straight-line stability, while toe-out provides for quicker steering response. For example, toe-in causes the wheels to roll in intersecting paths, which results in straight-line stability as the wheels stay straight. In this setup, the wheels absorb any irregularity without changing the direction of the vehicle. In contrast, toe-out results in slight disturbances that cause the wheels to roll in directions that proscribe a turn. Thus, a toe-out vehicle may be trying to enter a turn, instead of maintaining a straight path. Typically racing cars have at least some toe-out to promote enhanced turning ability, while street cars, or basic passenger cars, use some toe-in for straight-line stability.
Camber angle is another wheel alignment metric that car racing drivers are concerned with. Camber measures the angle of a wheel relative to the road surface. It is more simply the vertical tilt of a wheel. Camber is important because it can improve tire grip and thus provide better control of the vehicle, especially during turns. The camber of the wheel may affect the steering and suspension of the vehicle. A wheel has zero or neutral camber if the wheel is perfectly perpendicular with the level ground. If the top of the wheel tilts outward from the vehicle and the bottom slopes in, the wheel has positive camber. If the top of the wheel tilts in toward the vehicle and the bottom slopes outward, the wheel has negative camber. Typically, racing cars have some negative camber, while street cars have slightly positive camber. The more tire surface area that is in contact with the road, the better grip and stability the vehicle will have. Ideally, the tread surface of a tire is flat relative to the ground. However, because streets are typically not completely level, a slightly positive camber will provide better surface contact and grip. Thus, street cars, which are tuned primarily for straight-line stability, have wheels that are typically aligned with slightly positive camber. Racing car drivers however, may desire maximum cornering control or turning control, and thus may adjust the wheels to have negative or positive camber so as to compensate for tire deflection during cornering.
Camber is typically measured using a camber gauge. The gauge is set to zero and attached to the front hub of the wheel so that it is parallel to the surface of the wheel hub. The camber is then read from the camber gauge.
Toe measurement involves a more manual process. The traditional method of toe measurement used in car racing utilizes two manual measurements per wheel. The user first measures the distance between a reference marker, such as a string positioned parallel to the chassis centerline, and the rear edge of the wheel, and then measures the distance between the reference marker and the front edge of the wheel. The distance from the rear edge of the wheel is then subtracted from the distance from the front edge of the wheel. This difference is the toe. If the difference is negative, the measurement indicates that the wheels are toe-in. If the difference is positive, the measurement indicates that the wheels are toe-out.
While the traditional car racing method is typically used to align the wheels for racing cars, there are also several different types of other methods that use various devices to measure toe. Methods to measure toe are generally divided into the following four categories: (1) manual measurement relative to parallel strings or lasers fixed to the chassis (traditional racing method); (2) mechanical or optical calculation of total toe per axle by comparing positions of wheel fixtures; (3) electronic computation of camera images to determine wheel target positions; and (4) mechanical or optical measurement of individual toe angles by comparing wheel positions to an external reference system
Digital calipers may be used to manually measure toe relative to parallel strings or lasers fixed to the chassis in the traditional method. In this method, a first measurement is taken using digital calipers that are placed against the rear edge of the wheel, zeroed, and moved until it is aligned with the string or laser reference marker. The first measurement is then recorded and the digital calipers are zeroed again. A second measurement is taken using the digital calipers at the front edge of the wheel. The difference of the two measurements is the resulting toe.
While the traditional method continues to be used, the manual measurements involved in this method present several issues that make the process slow and prone to error. Two observational errors with each measurement may potentially occur—the measurement from the rear of the wheel to the reference marker and the measurement from the front of the wheel to the reference marker. Additionally, the person taking the measurement must do subtraction, which may also introduce mistakes in the value, sign or both. Even small errors in measurements can cause substantial effects in vehicle behavior due to the tire's high sensitivity to slip angle, which is the angle between the wheel's rotational plane and the direction in which it is moving. These issues make the manual process less reliable in high performance contexts such as racing car racing where the wheels frequently need aligning.
In the second method, mechanical or optical calculation of total toe per axle is calculated by comparing positions of wheel fixtures. In this method, a first device is attached to a first front wheel. A light beam is projected to a second device on the second front wheel and total toe is determined from the angle of reflection. A difference between this method and the traditional method used in car racing is that in the traditional method, individual toe relative to the chassis can be determined, while in this method only the angle between the two measured wheels is determined and the relative chassis centerline position remains undetermined.
The third method uses the electronic computation of camera images to determine wheel target positions. In one embodiment disclosed in U.S. Publication 20110179656 A1, active sensing heads with image sensors and at least one spatial relationship sensor for sensing a relationship between active sensing heads are mounted on each wheel to measure toe. Active sensing heads may include inclinometers for sensing tilt angles of the respective sensing head. This method is commonly used in street car shops. They are very accurate and can determine individual toe angles relative to the chassis centerline, but are bulky and expensive. As a result, they are not generally used by racing teams for car alignment during races.
The last method utilizes mechanical or optical measurement of individual toe angles by comparing wheel positions to an external reference system. In this method, toe is determined by measuring wheel and chassis positions relative to an external reference system. The external reference system may be a device or structure with known dimensions that is mounted to the front and back of the vehicle. In one embodiment, the external reference may be a string reference system where the string is attached to an external structure placed at the front of the vehicle so that the string is not attached to the chassis. In this method, the individual toe angles relative to the chassis can be determined through geometric calculations but are not measured directly against the chassis.
While there are various methods for measuring wheel alignment, in the racing context, a method that is reliable, accurate, and quick to perform is necessary. The method traditionally used in racing provides a quick way to check toe alignment, but an improvement is necessary in order to avoid potential errors in measurement accuracy which may be costly to performance.